Pretty sure you can. I did a 14-28 in 9-speed once. You can probably get the first position 14 from
http://www.SheldonBrown.com or
http://www.Branford bikes.com. Exactly what you can do with your 8-speed depends on what you've got to work with, how your cogs are assembled, which are individuals and which are fixed together, and if they can be separated.
Al